Polycarbonates are well known as tough, clear, highly impact resistant thermoplastic resins. However, these polycarbonates also possess a relatively high melt viscosity. Therefore, in order to prepare a molded article from polycarbonate, relatively high extrusion and molding temperatures are required. Various efforts throughout the years to reduce the melt viscosity while also maintaining the desired physical properties of the polycarbonates have been attempted. These methods include the use of plasticizers, the use of aliphatic chain stoppers, reduction of molecular weight, the preparation of bisphenols having long chain aliphatic substituents and various polycarbonate copolymers as well as blends of polycarbonate with other polymers.
With respect to plasticizers, these are generally used with thermoplastics to achieve higher melt flow. However, usually accompanying the plasticizer incorporation into polycarbonate compositions are undesirable features such as embrittlement and fugitive characteristics of the plasticizer.
Increased flow can be fairly readily obtained with the use of aliphatic chain stoppers, however, impact resistance as measured by notched Izod drops significantly. Embrittlement may also be a problem.
When utilizing a bisphenol having a lengthy aliphatic chain thereon, increases in flow can be observed. However, these are usually accompanied by substantial decrease in the desirable property of impact strength.
Various processes have been utilized to prepare polycarbonates with increased processability. When utilizing a copolyestercarbonate with an aliphatic segment, processes such as the pyridine solvent process of U.S. Pat. No. 3,169,121, have been utilized as well as processes utilizing diacid halides in an interfacial process sequence such as disclosed in U.S. Pat. No. 4,238,596 and U.S. Pat. No. 4,238,597. Additionally, high molecular weight aliphatic segments have been introduced into the polycarbonate (by interfacial methods) utilizing dicarboxylic endcapped polyisobutylene segments, see Mark and Peters U.S. Pat. No. 4,677,183 and U.S. Pat. No. 4,628,081. Additionally, a method of incorporating aliphatic dicarboxylic acids into polycarbonate is disclosed in Kochanowski, U.S. Pat. No. 4,286,083 wherein in an interfacial process the diacids are reacted together with a dihydric phenol and a carbonate precursor such as phosgene.
Although a standard interfacial process utilizing the chloride derivative of the saturated aliphatic alpha omega diacids can be employed to prepare the copolyestercarbonate, the availability of the diacid chloride starting materials is a problem. Aliphatic diacid chlorides are commerically available only in limited quantities and at a very high cost. Furthermore, even high purity diacid chlorides contain color contaminants which cause the final molded parts to display an unattractively high yellowness index. Therefore, attention was focused on the readily available, relatively inexpensive diacid starting materials. The previously mentioned Kochanowski patent was studied. The disclosure is directed to the usage of various aliphatic dibasic acids as disclosed at column 5, lines 13 through 22 in combination with a dihydric phenol and a carbonate precursor such as phosgene in an interfacial process. According to Kochanowski at column 6, lines 24 through 31, the reaction was carried out at a pH of between about 4.5 and 8.5, preferably between about 5.5 and 6.5 until the dibasic acid is consumed. The pH of the reaction is then raised to a value of between 9 and 11.5 to complete the reaction. The polyestercarbonate is isolated according to standard techniques, see column 6, lines 24 through 30 of Kochanowski. Experiments which followed the Kochanowski disclosure were conducted, Fontana et al., U.S. Pat. No. 5,025,081. 50% of adipic acid (10 mole percent) was incorporated into the polycarbonate backbone and yielded a 5 mole percent copolyestercarbonate. The preferred pH range disclosed in Kochanowski does not bring about complete incorporation of diacids into copolyestercarbonates in a reasonable time period. According to U.S. Pat. Nos. 4,286,083 and 4,983,706, it is possible to incorporate high molecular weight (C.sub.8-20) into polycarbonates. Lower molecular weight acids, such as adipic acid, are generally less expensive than the higher molecular weight acids, hence it is desirable to incorporate these lower molecular weight acids into polycarbonates.
A new process has been discovered which produces a reduced T.sub.g polyester polycarbonate derived from aliphatic dicarboxylic acids having from 4 to 8 carbon atoms. In order to obtain this reduced T.sub.g polyester polycarbonate, a stepwise pH range is followed in which the initial pH range is from 3 to 8, followed by raising the pH to a range of from 10 to 12 in subsequent stages. More specifically, low T.sub.g polyester polycarbonates can be made from low cost adipic acid, in a method using relatively short reaction times, for example, less than 11/2 hour.